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Lindsay Lohan continues to drink -- and drink often -- despite being sentenced this week to 90 days of lockdown rehab ... TMZ has learned.

Sources close to Lindsay tell TMZ she was at the bar of her hotel in Beverly Hills just hours after her court hearing on Monday ... knocking back vodka sodas. We're told Lindsay did not seem fazed by the irony of the situation.

It goes on -- we're told Lindsay went to a club in San Diego called FLUXX last night with a friend, arriving after midnight. Our spies say LiLo was wearing a Yankees cap and a hooded sweatshirt and sat at a VIP table ordering vodka on the rocks all night.

We're told Lindsay specifically request that vodka be served from a glass carafe ... this way the table would not have any alcohol bottles on it.

Sources close to Lindsay tell us she is adamant she does not have a substance abuse problem and doesn't feel she needs professional help.